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The Ginzburg temperature of ionic fluids revisited

Using the collective variables method we revisit the estimates of the Ginzburg temperature for the Coulomb-dominated models of ionic fluids. We consider the charge-asymmetric primitive model supplemented by short-range attractive interactions in the vicinity of the gas-liquid critical point. For this model, we derive the effective Ising-like $ϕ^{4}$-model Hamiltonian expressed in terms of the collective variables describing the fluctuation modes of the total number density. We obtain the explicit expressions for all the Hamiltonian coefficients within the framework of the same approximation, namely, the one-loop approximation which produces the mean-field critical parameters. Based on this Hamiltonian, we consistently calculate the reduced Ginzburg temperature $t_{G}$ for both the pure Coulombic model (a restricted primitive model) and the pure solvophobic model (a hard-sphere square-well model) as well as for the model parameters ranging between these two limiting cases. Contrary to the previous theoretical estimates, we obtain the reduced Ginzburg temperature for the pure Coulombic model to be about 20 times smaller than for the pure solvophobic model. For the full model including the short-range attractive and long-range Coulomb interactions, we show that $t_{G}$ approaches the value found for the pure Coulombic model when the strength of the Coulomb interactions becomes large enough. Our results suggest a key role of the Coulomb interactions in the crossover behaviour observed experimentally in ionic fluids as well as confirm the Ising-like criticality in the Coulomb-dominated ionic systems.